This invention relates to a process for removing evaporable components from polyamide melts at subatmospheric pressure and under steady agitation while simultaneously conducting a gaseous medium through the melt.
Polyamide-6, just after its production, contains a number of components which if they remain in the products manufactured from it lead to difficulties in the further processing and to a reduction in the quality of the end product. Here the content in monomers and oligomers plays a special role, whicy, by reason of an equilibrium reaction, runs to about 8 to 11% by weight. For most purposes of use of polyamide-6, it is required that the content of monomers and oligomers (also designated as extract content) lie below 2% by weight.
In spinning threads and fibers of polyamide-6 it is possible either to proceed from chips or to spin the polymerized melt directly. In the circuitous process using chips, the product, by reason of aqueous chip extraction and drying, has a desired, low extract content below 2%, but the expenditure for achieving this goal is very great both in respect to the apparatus needed and the energy requirements. Process steps include the production of chips, their extraction, drying and remelting. Therefore, additional devices are required for the production of the chips, extraction apparatus units, driers and extruders. The heat content of the melt is lost through the cooling in the chip production. Both in the drying and in the remelting of the chips energy is required.
In the direct spinning of polyamide-6, the expenditure on the installation as well as the energy requirement is appreciably less, but the spun product contains an extract content, depending on the procedure, of about 3% and above, too high for today's demands. In direct spinning the melt coming from the polymerization reactors is demonomerized in vacuum stages, in which the extract content recedes from ca. 10% by weight to ca. 3% by weight. In such vacuum stages generally the "wet wall" principle is employed, i.e., the melt is dispensed over the circumference of a vertical cylindrical container under high vacuum, flows in a thin layer down the container walls and collects in the container sump. There a large part of the monomers and oligomers evaporates. The effectiveness of the process is determined primarily by the layer thickness of the film, by the viscosity of the polymer, by the run-off length or residence time, the degree of the vacuum and the size of the sump. The layer thickness of the polyamide film should be as small as possible. However, a lower limit is dictated by the necessity of uniformly wetting the container wall, as well as by the size of the container and the desired maximum throughput. With known processes and apparatus of this type an extract content of ca. 3% by weight can be achieved.